Generally, in power cables the metallic conductor is covered with an inner polymeric semiconductive layer, a polymeric insulation layer, an outer polymeric semiconductive layer, a metallic shield and finally a polymeric jacket.
A wide variety of polymeric materials have been utilized as electrical insulating and semiconductive shield materials for cables in numerous applications. In order to be utilized in services or products, where a long-term performance is required, such polymeric materials, in addition to having suitable dielectric properties, must also be enduring and must substantially retain their initial properties for safe performance over many years of service. A semiconductive polymer composition must fulfill a number of properties, of which the electrical properties are most vital.
It is known that the space charge performance of cables can be influenced by the selection of the components in the semiconductive material. Space charge is an accumulation of electrical charges (electrons, holes and ions) inside the insulation leading to electrical field distortion. They emanate from components inside the insulation or from injection of electrons from the semiconductive layers. Space charges trapped in high voltage insulation systems (i.e. polymeric power cables) can significantly alter the internal electrical field distribution, possibly leading to premature failure of the system at stresses well below anticipated or design values.
Efforts have been made to improve semiconductive polymer compositions, especially to reduce the space charge effects by maintaining other important properties, for example surface smoothness and good processability. Thereby semiconductive polymer compositions have been used which include an ethylene-acrylate copolymer, carbon black, stabiliser and organic peroxide crosslinking agent.
In the following some concepts shall be defined:
The melt flow rate (MFR) is measured in g/10 min. of the polymer discharged through a defined die under specified temperature and pressure conditions and is a measure of the viscosity of the polymer which in turn for each type of polymer is mainly influenced by its molecular weight, but also by its degree of branching. The melt flow rate measured under a load of 2.16 kg (ISO 1133) is denoted as MFR2. In turn, the melt flow rate measured with 21.6 kg is denoted as MFR21.
As a further characteristic, the molecular weight distribution (MWD) which is the relation between the number of molecules in a polymer and the molecular weight of the individual molecules has to be considered. The width of the distribution is given as a number defined as the ratio (Mw/Mn) between the “weight average molecular weight” Mw divided by the “number average molecular weight” Mn.
EP 1 065 672 A2 (UNION CARBIDE CHEM PLASTIC) discloses a composition comprising an olefin polymer, such as a copolymer of ethylene and unsaturated esters with an ester content of at least about 5% per weight based on the weight of the copolymer, and carbon black of about 25% to about 45% per weight based on the weight of the composition, with strictly defined properties, such as particle size of at least about 29 nm and a tint strength of less than about 100%. However, the application gives no information how the space charge properties can be influenced.
EP 0 644 558 A1 (Alcatel Cable) describes an insulative structure for cables, comprising at least one first semiconducting layer that is contiguous and coaxial with the core of the cable and is surrounded by a second electrically insulating layer which is itself covered by a third semiconducting layer, characterised in that the said semiconducting layers are exclusively composed of a matrix, comprising non-polar polymers whose components have a molecular weight (molar mass) greater than 1000.
CA 2145366 (BICC Cables Corp) discloses a semiconductive shield compositions containing a linear, single-site catalysed polymer formed by polymerising ethylene with at least one comonomer selected from C3 to C20 alpha-olefins; a carbon black selected from furnace carbon blacks that contain ash and sulphur in amounts of 50 ppm or less and have crystal dimensions La and Lc of 30 A or less, acetylene carbon blacks, and furnace carbon blacks having an ASTM grad of N-351; and a crosslinking agent. The ethylene copolymer has a narrow molecular weight distribution due to the single-site catalyst and is therefore not multimodal.
U.S. Pat. No. 5,246,783 (Exxon Chemical Patents, Inc.) describes a semiconductive composition comprising ethylene copolymer having a density of 0.86 g/cm3 to 0.96 g/cm3 and a molecular weight distribution Mw/Mn of 1.5 to 30. However, the disclosure does not use a multimodal ethylene copolymer.
It is known that polymers produced by a single-site catalyst have a narrow molecular weight distribution (MWD) which is detrimental to known extrusion processes. For good extrudability, however, a broad molecular weight distribution is required.
Therefore, the problem which has to be solved is to provide a semiconductive polymer composition with both improved space charge performance and good processability leading to long lifetime of the cables.